Example Of Electrophilic Substitution Research Articles

Bromination and Iodination of 6H,12H‐5,11‐Methanodibenzo[b,f][1,5]diazocine: A Convenient Entry to Unsymmetrical Analogs of Tröger's Base

Abstract6H,12H‐5,11‐Methanodibenzo[b,f][1,5]diazocine can easily be prepared from aniline and paraformaldehyde. Its reactions with either N‐bromosuccinimide (NBS) or with iodine monochloride (ICl) in the presence of a suitable activator smoothly afford 2‐bromo‐ and 2‐iodo‐ derivatives, respectively. The combination of these halogenation methods provides access to the 8‐bromo‐2‐iodo derivative, which is an interesting and otherwise inaccessible intermediate for the synthesis of unsymmetrical analogs of Tröger's base. The reported halogenations represent the first examples of electrophilic substitution in 6H,12H‐5,11‐methanodibenzo[b,f][1,5]diazocine. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Electrophilic aromatic substitution. Part 36. Protiodetritiation of some annelated meta-cyclophanes: effect of ring-buckling on reactivity, and the first example of electrophilic substitution through a hole

A series of 1,3-brigded naphthalenes in which the alkyl bridge contains ten, eight, or seven methylene groups have been prepared with tritium located at either the 2- or the 4-position. Their rates of detritiation in anhydrous trifluoroacetic acid at 70 °C have been measured. The partial rate factors for exchange at the 4-position increase as the size of the bridge decreases viz. 1.28 × 107([CH2]10) 2.71 × 107([CH2]8), 2.97 × 107([CH2]7); cf. 1.15 × 107 for 1,3-dimethylnaphthalene, the corresponding σ+ values being –0.812, –0.849, –0.854, and –0.807. This is consistent with increased loss of ground-state aromaticity as the aromatic ring becomes more buckled. At the 2-positions the partial rate factors are 2.37 × 105([CH2]10), 2.92 × 105([CH2]8), and 1.50 × 105([CH2]7); cf. 2.82 × 105 for 1,3-dimethylnaphthalene, the corresponding σ+ values being –0.614, –0.625, –0.592, and –0.623. The results here reflect the opposing effects of increased reactivity due to loss of ground-state aromaticity, and steric hindrance to exchange as the bridge is made smaller. Because of the symmetry of the reaction pathway for hydrogen exchange, reaction at the 2-positions involves attack of the electrophile at one face of the loop formed between the alkyl chain and the aromatic ring, and departure of the leaving group (triton) from the opposite face, i.e. substitution occurs through a ‘hole’. This is the first example of electrophilic substitution at an enclosed site. The fact that steric effects in both reagent and product in hydrogen exchange are almost identical is postulated as a primary reason for the extremely low steric hindrance generally observed in the reaction. The effect of fusing a bicyclic substituent onto an aromatic ring has been determined for the first time. For hexahydrophenanthrylene and hexahydroaceanthrylene (naphthalenes with bicyclic substituents attached to the 1-, 2-, and 3-positions) the average partial rate factor for detritiation of the ‘4’-positions is 5.7 × 107, and σ+ is –0.886. This is greater than calculated on the basis of additivity of the alkyl group substituent effects and is attributed to steric facilitation of hyperconjugation. The recent suggestion (based on structure counts) that the exalted reactivities of helicenes (and by implication of other aromatic systems containing bent benzene rings) is not due to loss of ground-state stability is shown to be invalid.

First example of electrophilic substitution by addition–elimination (σ-substitution) in pyrroles

Reaction of 1-methylpyrrole with N-chloroacetanilide leads to the incorporation of an acetanilide moiety in the pyrrole ring by addition–elimination.

The first example of electrophilic substitution on a benzannulene

Electrophilic substitution (nitration, acylation) proceeds readily in the macro ring of the benzannelated [14]annulene 1 to give products similar to that of the non annulated parent 6.

The problem of the stereochemistry of the reaction of symmetrical organomercury compounds with mercuric halides

1. The stereochemistry of the reactions of symmetrical, α-mercurated L-menthyl phenylacetate and mercuribis-α,α′-camphor with mercuric bromide in acetone solution has been studied. Both reactions are examples of electrophilic substitution at a saturated carbon atom. 2. It is shown that the first reaction proceeds with retention of configuration at the carbon atom in question. The racemization observed at 56° is a secondary process. 3. It is not possible to elucidate the stereochemistry of the reaction between mercuribis-αα′-camphor and mercuric acetate because the 3-bromomercuricamphor formed racemizes even in the cold by the action of mercuric acetate under the experimental conditions. 4. By a study of the reaction of the symmetrical, α-mercurated L-menthyl ester with an equimolar quantity of hydrogen bromide the conclusion, drawn previously [1], that the symmetrization of organomercury salts proceeds with retention of the configuration at the saturated carbon atom was confirmed.